Modern electronics and communication systems may require accurate output power control for effective transmission of data. A variable gain stage may be used to control output power. The variable gain stage can work in an open loop mode (i.e., without feedback), or a closed loop mode (i.e., with feedback). The variable gain stage may be implemented via Variable Gain Amplifiers (VGA) or Programmable Gain Amplifiers (PGA).
When a VGA controls the output power directly in open loop mode, factors pertaining to a control port of the VGA, such as slope linearity and step accuracy specifications, may prove critical during implementation. For example, the step accuracy and the slope linearity specifications of a control port of a VGA can be difficult to meet due to fine step requirements for accurate output power control. In certain implementations, a ramping signal may be provided for power change instead of a sharp step response.
Furthermore, a VGA generally implements an analog gain stage, which makes control port specifications difficult to implement. Furthermore, if the VGA is controlled in a closed loop mode, the control slope of the VGA varies, thereby varying the closed loop bandwidth and making the closed loop non-linear. Due to this, transient performance of the VGA may be affected and phase margin of the VGA may be reduced. An additional issue with closed loop systems is that there is a trade off between speed of locking and accuracy of step.
In certain systems, to control the output power, slope characteristics of a VGA may be controlled either by tight analog design specifications or by gain control linearization techniques. Tight analog design specifications may require complex circuitry, which can be cost intensive or current consuming. Gain control linearization techniques may involve the use of additional compensation circuitry or use of digital linearization techniques such as a look-up table which may require additional die area and current consumption. In addition, certain systems may control gain step accuracy of the VGA by precision control of currents or voltages, which also may be a function of circuit complexity.
Circuit complexities in the design of a VGA could demand additional circuit design efforts and increased current consumption. Furthermore, all of the above mentioned techniques are typically valid for a single temperature and supply voltage range due to analog VGA circuitry. Therefore, additional compensation circuitry may also be required across various operational variations.